The present invention relates to a stencil printer.
Stencils for thermal printing include one having a thermoplastic resin film, a porous thin sheet or similar base permeable to ink and adhered to the resin film, and an anti-stick layer formed on the resin film for preventing the film from sticking to a thermal head. The porous sheet is, in many cases, implemented by flax fibers or a mixture of flax fibers, synthetic fibers, and wood fibers.
The above conventional stencil, however, has the following problems (a) and (b) left unsolved because the fibrous base just overlies the resin film.
(a) A great amount of adhesive gathers in the from of webs in regions where the overlapping portions of fibers and the resin film contact each other, obstructing the perforation of the stencil by the thermal head.
(b) Fibers themselves check the passage of ink and make printing irregular.
To solve the above problems (a) and (b), some different stencils for thermal printing have been proposed in the past. Japanese Patent Laid-Open Publication No. 3-193445, for example, teaches a porous thin sheet or base implemented by fibers as thin as 1 denier. This kind of base solves the problem (b), but cannot solve the problem (a).
Japanese Patent Laid-Open Publication No. 4-7198 discloses a printing method using a stencil produced by applying a mixture solution of fine particles of polymer dispersion and colloidal silica to the surface of a film and then drying it to form a porous layer. The stencil is perforated by a master making machine Print Gokko (trade name) available from Riso Kagaku Corporation to thereby make a master. The master is used to print images on papers with ink HG-4800 available from EPSON. The above porous layer, however, does not allow ink to smoothly pass therethrough and cannot implement satisfactory image density when use is made of conventional ink for thermal printing. Moreover, the porous layer itself lacks a sufficient heat insulating ability and prevents the stencil from being desirably perforated.
Japanese Patent Laid-Open Publication No. 54-33117 proposes a stencil consisting substantially only of a heat-sensitive resin film, i.e., not including a base. While this kind of stencil solves the previously stated problems (a) and (b) at the same time, it brings about other problems, as will be described hereinafter.
When the film constituting the stencil is 10 xcexcm thick or less, the stencil fails to have sufficient stiffness and cannot be easily conveyed. In light of this, Japanese Patent Publication No. 5-70595 proposes to wrap an elongate film around a print drum without cutting it and cause the entire film to rotate together with the print drum during printing. This scheme, however, increases the turning moment because the film and a master attaching and detaching unit rotate together with the print drum during printing. This, coupled with the noticeable offset of the center of gravity from the axis of rotation, requires a printer to have a heavy, bulky configuration.
On the other hand, when the film is 5 xcexcm thick or more, its heat sensitivity is reduced and obstructs perforation by the thermal head. Moreover, heat applied from heating means is transferred to a platen via the stencil and lost in a substantial amount, limiting energy available for the perforation of the film. Japanese Patent Laid-Open Publication No. 10-230690 discloses a stencil made up of a thermoplastic resin film and a porous resin film or base provided on one surface of the resin film, i.e., a stencil without a filler.
Specifically, the porous resin film included in the above stencil is formed by precipitating a resin solved in a solvent and, e.g., solidifying it. Comparing the film to a floor, the porous resin film is a wall-like film implemented as an assembly of a number of cells with or without a ceiling, a foam-like film implemented by an assembly of open cells, or a film implemented by an assembly of resin in the form of particles or fibers. The cells may be fully closed or partly open. Openings appear on the surface of the porous resin due to the bursting of cells occurring in the drying step.
Open cells, a resin in the form of particles or fibers and cells without a ceiling, which constitute the porous resin film in combination, are connected together. This configuration provides the resin film with sufficient tensile strength and stiffness and thereby provides the stencil with sufficient tensile strength while allowing it to be smoothly conveyed.
Generally, the porous resin film has a mean cell size of 1 xcexcm or above, but 50 xcexcm or below. Mean cell sizes less than 1 xcexcm obstruct the smooth passage of ink. Should ink with low viscosity be used to allow it to pass through the resin film in a sufficient amount, it would blur images and would ooze out from the sides of the print drum and the trailing edge of a master wrapped around the drum. In addition, the void content of the resin film would decrease and would therefore further obstruct the perforation by the thermal head.
Mean cell sizes greater than 50 xcexcm reduce the ink regulating effect available with the porous resin film. As a result, ink is forced out from the print drum to a paper or similar recording medium in an excessive amount, smearing the rear surfaces of papers and blurring images.
The porous resin film should only have a number of voids therein side and on the surface thereof. To promote the passage of ink, the voids should preferably be communicated to each other in the direction of thickness of the film and extend, comparing the film to the floor, throughout the ceiling. Alternatively, at the boundary between the porous resin film and the thermoplastic resin film, the former may cover the latter so long as it does not obstruct perforation by the thermal head. The resin constituting the porous resin film so covering the thermoplastic resin film should generally be 7 xcexcm thick or less inclusive of the thermoplastic resin film, although dependent on the kind of the resin, heat sensitivity of the thermoplastic resin film, etc.
The total area of openings having diameters of 5 xcexcm or above in terms of a true circle, as measured on the surface of the porous resin film, is 4% to 80%, preferably 10% to 60%, of the entire surface area. If this radio is less than 4%, the resin film is apt to obstruct the perforation by the thermal head and the passage of ink. If the ratio is greater than 80%, the tensile strength and stiffness of the resin film decrease.
The porous resin film is entirely different in structure from the conventional porous portion of a stencil for thermal printing. Specifically, solid portions included in the porous resin film have various shapes including a rod-like shape, a spherical shape and a branch-like shape. The configuration of the solid portions is determined by conditions for the fabrication of the resin film, e.g., the kind of the resin, the solid content of a liquid, the kind of a solvent, the amount of deposition of a resin solution, the temperature of the resin solution, temperature for drying the resin solution, and ambient temperature and humidity for application. Among them, the temperature of the resin solution and ambient temperature and humidity for application have critical influence on the configuration of the solid portions.
For example, when the temperature of the resin solution is 10xc2x0 C. or below, the resin solution easily gels and is difficult to apply. Conversely, when the above temperature exceeds 30xc2x0 C., it is difficult to form the porous resin film. It follows that the temperature for application should also preferably be between 10xc2x0 C. and 30xc2x0 C. The ambient humidity for application above 50% RH would cause the surface of the thermoplastic resin film to adsorb a great amount of water and would thereby lower wettability with respect to the solution, weakening adhesion between the porous resin film and the thermoplastic resin film.
The stencil with the above porous resin film realizes attractive images when used with a stencil printer PREPORT VT3820 (trade name) available from Ricoh Co., Ltd. and ink VT60011 (lot No. 960604-22) (trade name) for PREPORT and when perforated and used for printing (three printing speeds) in a 20xc2x0 C., 60% RH environment with a 7% greater pulse width than in the standard state. Images are attractive when image density is 0.7 to 1.3, preferably 0.9 to 1.25, as measured by a densitometer RD914 available from Macbeth. The porous resin film is clearly distinguishable in structure from the resin film of Laid-Open Publication No. 4-7198 mentioned earlier. The above ink for PREPORT has viscosity of 150 Poise at 20xc2x0 C., as measured by a densitometer HAAKE CV20 and a rotor PK 30-4 at a share rate of 20 (1/S).
When ink for an ink jet printing system is applied to the master including the porous resin film, it is transferred to a paper in an excessive amount and blurs the resulting image.
The total area of openings having diameters of 5 xcexcm or above in terms of a true circle, as measured on the surface of the porous resin film, is 50% or above, but 70% or below, of the entire opening area. If this radio is less than 50%, the resin film is apt to obstruct the perforation by the thermal head and the passage of ink.
The porous resin film or base has a thickness between 5 xcexcm and 100 xcexcm, preferably between 6 xcexcm and 50 xcexcm. Thickness less than 5 xcexcm cannot implement sufficient film strength and makes it difficult for the porous resin film to remain at the rear of perforated portions. This renders control over the amount of ink transfer impracticable and aggravates the smearing of the rear of surfaces of prints. Thickness greater than 100 xcexcm is apt to obstruct the passage of ink. The regulation of the amount of ink transfer available with the porous resin film is more promoted as the film thickness increases, so that the amount of ink transfer to a paper can be controlled in terms of the thickness of the above film. When the thickness after the application is greater than the target value, the porous resin film can be thinned to the target value by a calender or similar suitable means.
When the mean cell size of the porous resin film is 20 xcexcm or less, the passage of ink is more obstructed as the thickness of the resin film increases. It is therefore possible to control the amount of ink transfer to a paper on the basis of the thickness of the porous resin film. The thickness should preferably be uniform; otherwise irregular printing occurs. The thickness is measured without any substantial load or with an extremely light load.
The amount of deposition of the porous resin film or base ranges from 0.5 g/m2 to 25 g/m2, preferably 2 g/m2 to 15 g/m2. Amounts greater less than 0.5 g/m2 prevent the porous resin film from having sufficient strength while amounts greater than 25 g/m2 obstruct the passage of ink and thereby degrade image quality.
The porous resin film has density between 0.01 g/cm3 and 1 g/cm3, preferably between 0.1 g/cm3 and 0.5 g/cm3. Density below 0.01 g/cm3 prevents the resin film from having sufficient strength while density above 1 g/cm3 obstructs the passage of ink and thereby degrades image quality. A master for thermal printing should preferably have stiffness of 5 mN or above in terms of bending rigidity, as measured by Lorentzen stiffness tester. Bending rigidity below 5 mN makes the conveyance of the stencil on the printer difficult.
When the thermoplastic resin film of the above stencil is perforated by 20% in terms of the open ratio, air flows through the resin film at a range of 1.0 cm3/cm2 sec to 157 cm3/cm2 sec, as measured by a permeability tester. The above open ratio refers to, when a solid image is formed in a stencil for thermal printing by, e.g., a thermal head, a laser or a flash lamp, the ratio of the total area of through holes formed in the thermoplastic film of the stencil to the unit area of the solid image.
When the open ratio is less than 20%, ink with extremely low viscosity must be used in order to guarantee desired image density. This kind of ink deteriorates the uniformity of a solid image and the reproducibility of thin lines when it comes to a stencil printing system. When permeability is less than 1.0 cm3/cm2 sec, the passage of ink is deteriorated; ink with low viscosity used for implementing a sufficient amount of ink transfer would blur an image and would be forced out from the sides of the print drum and the trailing edge of the master wrapped around the drum. Moreover, the above permeability often reduces the void content of the porous resin film and obstructs the perforation by the thermal head.
Permeability above 157.0 cm3/cm2 sec degrades the ink regulating effect available with the porous resin film and causes ink to be forced out from the print drum to a paper in an excessive amount and brings about offset and blurring.
To render the perforation of the thermoplastic resin film by the thermal head more effective, at least part of the porous resin film contacting the thermoplastic resin film should preferably soften at 150xc2x0 C. or below.
To print an image on a paper, ink is passed through perforations formed in the thermoplastic resin film by the thermal head. Ink cannot be passed through closed cells. However, this problem is solved because ink for use in a thermal stencil printer is generally a W/0 emulsion and because the film structure of the porous resin film is partly destroyed by the above components. Cells should preferably be not closed.
To form cell, use may be made of substances that do not exhibit the above characteristic alone, but exhibit it when combined. If desired, part of the film may be destroyed either mechanically or chemically during or after the formation of cells. In practice, to produce a stencil for thermal printing, it is preferable to form a porous resin film formed of foam on a thermoplastic resin film.
For the major component of the porous resin film, there may be used any one of plastics including polyethylene, polypropylene, polybutene, styrene resin, polyvinyl chloride, polyvinylidene chloride, polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, polyvinyl acetal, vinyl chroride-vinyl acetal copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, styrene-acrylonitrile copolymer and other vinyl resins, polyacrylonitrile, polyacrylic acid plastics, diene plastics, polybutylene, nylon and other polyamides, polyester, polyphenylene oxide, (meta)acrylic acid ester, polycarbonate, polyacetal, fluorocarbon resin, polyurethane plastics, natural plastics, natural rubber plastics, thermoplastic elastomers, acetyl cellulose, acetylbutyl cellulose, acetylpropyl cellulose andother cellulose derivatives and other microbial plastics, and copolymers containing such polymers. Use may also be made of various fatty acids, waxes and other carbohydrates, and various proteins.
To form the porous resin film of the stencil, there may be effected steps of applying a fluid resin solution (composition) containing foam, a fluid resin solution (composition) containing one of two or more components which generate a gas on contacting each other or a fluid resin solution (component) in which a gas is dissolved at 1 atmosphere or above to the thermoplastic resin film and then drying it.
The stencil with the above porous resin film is free from the problems (a) and (b). However, there arises another problem that when relatively thick papers are used, their edges contact the same portion of the stencil or master wrapped around the print drum. This, coupled with the fact that a press roller presses such papers against the print drum, stress acts on the master. As a result, the porous resin film partly comes off the thermoplastic resin film and remains on the print drum in the form of a small piece. This piece remains on the print drum even after the master is replaced with a new master. At the time of the next printing executed with the new master wrapped around the print drum, the above piece left on the print drum blocks ink and thereby causes an image to be locally lost.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 4-105983, 4-185377, 8-58216, 8-332785 and 9-58104 and U.S. Pat. Nos. 5,843,560 and 5,908,687.
It is therefore an object of the present invention to provide a stencil printer capable of preventing a porous resin film or base forming part of a master wrapped around a print drum from partly coming off and remaining on the drum when use is made of relatively thick papers, thereby obviating the local omission of an image.
A stencil printer of the present invention includes a porous hollow cylindrical print drum rotatable with a perforated master having a porous resin film as a base wrapped therearound. A pressing member is selectively movable into or out of contact with the print drum for pressing a recording medium fed from a medium feeding device against the print drum. As a result, ink is transferred from the inside of the print drum to the recording medium via perforations formed in the master. The pressing member is implemented as a roller having surface hardness of 18xc2x0 or less (JIS (Japanese Industrial Standards)-A). Alternatively, the pressing member may be implemented as a roller having surface hardness of less than 15xc2x0 (JIS-A).